Dependency Injection Andres Käver, IT College 2016/2017 Spring

Dependency Inversion Principle Theory was first formalized by Robert C. Martin, 1996 High-level modules should not depend on low-level modules. Both should depend on abstractions. Abstractions should not depend upon details. Details should depend upon abstractions Example from life – myriad of chargers and small scale electronic devices

Dependency Inversion Every module is with its own interface Higher module has to support them all separately Modules are not reusable Higher module Interface Interface Interface Lower module Lower module Lower module

Dependency Inversion Higher module specifies the required interface All modules depend of the same interface Modules are reusable Interface Higher module Lower module Lower module Lower module

Inversion of Control Software development pattern Implements the Dependency Inversion principle Control over how different systems interface with each other Control over program events (command line vs graphical UI) Control over dependency creation and binding

Dependency Injection Implementation of IoC (DIP -> IoC -> DI) Dependency creation and resolving of them is moved outside of dependent class Injector Class Interface Class Interface Dependency Dependency

DI - Caution Internal principles of class behavior will be exposed Dependencies are created before they are actually needed Don’t overuse – is it needed for everything?

DI – Meta example Using constructor IRepo xmlrepo = new XMLRepo(); Kontroller knt = new Kontroller(xmlrepo); public class Kontroller{ private readonly IRepo repo; public Kontroller(Irepo repo){ this.repo = repo; }

DI – Meta example MVC During testing you can supply different implementation of repository In regular use default constructor is used and dependency is created public class PersonController:Controller{ private readonly IRepo _repo; public PersonController (Irepo repo){ _repo = repo ?? new PersonRepository(); } public PersonController() : this.PersonController(null) { }

Dependency Injection Container Framework Registers interfaces and their implementations Resolves dependencies and provides (injects) correct implementation Lifecycle management of created objects Several implementations, widely used are Ninject – Open Source Unity - MS official solution ASP.NET Core – has built-in lightweight solution out-of-the-box

Demo – console application public class PetOwner { private readonly IDog _dog; public PetOwner(IDog dog) _dog = dog; } public int Walkings() return _dog.WalkingCount; public void Walk() Console.WriteLine(_dog.Walk()); public interface IDog { string Walk(); int WalkingCount { get; set; } } public class DogPoodle : IDog public string Walk() WalkingCount++; return "Walking with Poodle "+WalkingCount.ToString()+" times!"; public int WalkingCount { get; set; }

Demo – console application static void Main(string[] args) { var kernel = new StandardKernel(); kernel.Bind<IDog>().To<DogPoodle>(); var dogKeeper = kernel.Get<PetOwner>(); dogKeeper.Walk(); Console.ReadLine(); }

Demo Asp.Net MVC Nuget Ninject.MVC5 Register your dependencies here \App_Start\NinjectWebCommon.cs private static void RegisterServices(IKernel kernel) { kernel.Bind<IPersonRepository>(). To<PersonRepository>(). WithConstructorArgument("context", new ContactsContext()); }

Demo Asp.Net MVC Controller public class PersonController : Controller { private readonly IPersonRepository _repo; public PersonController(IPersonRepository repo) _repo = repo; }